1. Field of the Invention
The present invention relates to an image pickup apparatus and, more particularly, to an image pickup apparatus having a white balance adjusting means.
2. Related Background Art
Conventional white balance adjusting devices of image pickup apparatuses are classified into an outer measure mode device for performing white balance adjustment by an output signal from an external color measure sensor shown in FIG. 1 and a through-the-lens (to be referred to as a TTL hereinafter) device for performing white balance adjustment in accordance with an output signal from an image pickup element, as shown in FIG. 2. Conventional examples will be described with reference to FIGS. 1 and 2.
FIG. 1 is a block diagram of a conventional outer measure mode device. This device includes an image pickup optical system 1a, a solid-state image pickup element 1, a luminance signal processing unit 2, a chromaticity signal processing unit (to be referred to as a chrominance signal processing unit hereinafter) 3, R and B gain control units 4 and 5, differential amplifiers 6 and 7, a modulation unit 8, an adder 9, a color temperature sensor 10, and a control signal derivation unit 11. The solid-state image pickup element 1 is an image pickup element for converting optical information into an electric signal. The luminance signal processing unit 2 performs appropriate processing of an output from the image pickup element 1 and derives a luminance signal Y. The chrominance signal processing unit 3 performs appropriate processing of the output from the image pickup element 1 and derives a low-frequency luminance signal Y.sub.L and chrominance signals R and B. The R and B gain control units 4 and 5 control levels of outputs R and B from the chrominance signal processing unit 3 and output signals R.sub.1 and B.sub.1. The differential amplifier 6 derives a color difference signal R-Y from the luminance signal Y.sub.L and the chrominance signal R.sub.1. The differential amplifier 7 derives a color difference signal B-Y from the luminance signal Y.sub.L and the chrominance signal B.sub.1. The modulation unit 8 derives a modulation signal defined in NTSC or PAL scheme in accordance with the color difference signals R-Y and B-Y. The adder 9 derives a predetermined video signal from the output Y from the luminance signal processing unit 2 and an output from the modulation unit 8. The color temperature sensor 10 comprises a color measure sensor arranged independently of an image pickup element to measure a color temperature of a light source for illuminating an object. The control voltage derivation unit 11 derives a voltage for controlling amplifier gains of the R and B gain control units 4 and 5 in accordance with an output from the color temperature sensor 10.
An operation of the above circuit arrangement will be described with reference to FIG. 1.
A Y signal is derived by the luminance signal processing unit 2 in accordance with an output from the image pickup element 1, and the Y.sub.L, R, and B signals are obtained by the chrominance signal processing unit 3. A color temperature of the light source which illuminates the object is measured by the color temperature sensor 10, and a control voltage to be applied to the R and B gain control units 4 and 5 for correcting white balance is obtained from the control voltage derivation unit 11. The R and B gain control units 4 and 5 output the color signals R.sub.1 and B.sub.1 whose white balance is corrected. A predetermined video signal having the adjusted white balance is derived from the signals Y, Y.sub.L, R.sub.1, and B.sub.1 through the differential amplifiers 6 and 7, the modulation unit 8, and the adder 9.
FIG. 2 is a block diagram of a conventional TTL mode device. Blocks 1 to 9 correspond to the identical blocks in FIG. 1. Averaging units 12 and 13 comprise low-pass filters for averaging the color differential signals R-Y and B-Y and converting them into DC voltages. In accordance with the average signals from the averaging units 12 and 13, a control voltage derivation unit 14 derives a control voltage applied to the R and B gain control units 4 and 5 for correcting white balance.
An operation of the TTL mode device described above will be described below.
Operations of the blocks 1 to 9 are the same as those in FIG. 1. The R-Y and B-Y signals averaged for one frame or several frames by the averaging units 12 and 13 are compared with a specific voltage corresponding to the zero level of the color difference signals by the control voltage derivation unit 14 so as to determine whether the R-Y and B-Y signals have levels higher or lower than the zero level. The control voltage derivation unit 14 outputs a control level so as to set the R-Y and B-Y levels to be closest to the zero level. This control voltage is input to the R and B gain control units 4 and 5 to adjust the white balance.
Still another conventional system as an addition system which is a combination of the outer measure mode and the TTL mode is available in addition to the above two conventional systems.
FIG. 3 is a block diagram showing this addition system.
Referring to FIG. 3, blocks 1 to 14 identical to those in the conventional arrangements in FIGS. 1 and 2. The control voltage derivation unit 11 is referred to as a first control voltage derivation unit, and the control voltage derivation unit 14 is referred to as the second control voltage derivation unit. Adders 27 and 28 add a control voltage from the first control voltage derivation unit 14 and a control voltage from the second control voltage derivation unit 14 at predetermined ratios. The sum voltages from the adders 27 and 28 are used to adjust the white balance.
In the conventional arrangements, a signal from the image pickup element 1 and a signal from the color temperature sensor serving as a color measure sensor 10 arranged independently of the image pickup element are added at a predetermined ratio to appropriately adjust the white balance.
Still another white balance adjustment scheme is also proposed in Japanese Laid-Open Patent Application No. 63-314424 filed by the present applicant. According to this scheme, one of two color measure methods is selected by a detection output to adjust the white balance.
In the above conventional outer measure mode in FIG. 1, precision of white balance adjustment is greatly degraded when light sources which illuminate an image pickup apparatus body and an object due to large separation of the image pickup apparatus body from the object or when a photographic condition is given as a sunset glow.
In the conventional TTL mode in FIG. 2, when the most of the object area is given as a chromatic color area or when most of the frame is occupied with a monotone, precision of white balance adjustment is degraded because the monotone tends to be corrected to white.
In the addition scheme in FIG. 3, although precision tends to be improved by adding the control voltages derived from the outer measure and TTL modes to each other so as to compensate for drawbacks of these conventional schemes, white balance adjusting effects unique to these conventional schemes are mutually cancel each other to degrade the precision. In the example described in Japanese Laid-Open Patent Application No. 63-314424, this scheme serves as a remedy only when an environmental condition is dark. This scheme is not suitable for white balance adjustment when a scene is bright.